The present invention relates to a process for removing protective coatings and/or bonding layers from a base metal, and more particularly, to a process for removing gamma prime bonding layers, intermetallic protective coatings, and metallic protective coatings from a part such as a gas turbine engine component part.
Protective coatings and bonding layers are widely used in modern gas turbine engines. The use of such protective coatings and bonding layers permits a designer to specify structural materials of high strength without having to be particularly concerned with the surface stability of the materials. Intermetallic coatings, and metallic coatings in particular are used on metal parts which encounter severe operating conditions at elevated temperatures. Such parts include gas turbine parts such as, the burner assembly, turbine vanes, and blades. Bonding layers are used to achieve a good bond between a base metal and a protective coating between which an adequate bond might not otherwise be obtained.
Various situations exist in which these protective coatings and/or bonding layers need to be removed. One such situation is when a part is crack damaged; the coating needs to be removed so that the part may be cleaned, repaired, recoated, and returned to service. Another situation arises upon improper application of the original coating; the coating needs to be removed so that a new coating may be applied. A third situation arises as the coating becomes worn while in service. The coating must be removed and a new coat applied.
Currently, these coatings are removed by the use of acid baths. U.S. Pat. No. 3,948,687 discloses an aqueous HF-HNO.sub.3 stripping bath in which CrO.sub.3 is also present for removing aluminized cases. The stripping bath operates at 85.degree. F. The aluminized case dissolves in the bath and the base metal is not significantly attacked.
Additionally, U.S. Pat. No. 4,176,433 discloses a more detailed process for chemically stripping an aluminide protective coating from the internal and external surfaces of a salvageable vane. The part is grit-blasted and then immersed in an agitated nitric acid solution at 75.degree. to 90.degree. F. for four hours. The part is then wet abrasive-blasted and immersed in an agitated nitric acid solution at 75.degree. to 90.degree. F. for four hours. The wet abrasive-blasting and immersion in acid are repeated until the coating is removed.
Fluorocarbon cleaning of crack damaged metal parts is known in the art. U.S. Pat. Nos. 4,188,237; 4,324,594; 4,328,044; and 4,405,379 disclose processes for cleaning crack damaged stainless steel, superalloy, solid solution superalloy, and gamma prime hardened nickel alloy parts which render the parts braze repairable. The preferred cleaning process involves a three-stage procedure to eliminate the passivating oxides from the metallic surface. In stage I, a cleaning atmosphere of carbon, oxygen, hydrogen, and fluorine between 450.degree.-800.degree. C., converts noble oxides on the metallic surface and in the cracks to their fluorides. During stage II, the atmosphere is maintained to draw Al and Ti from the surface by diffusion. In stage III, a predominantly hydrogen atmosphere between 750.degree. and 1000.degree. C., converts the crystalline non-volatile fluorides to their conjugate metals. This fluorocarbon cleaning process avoids operation within the sooting range, i.e., the point at which carbon precipitates from the gas phase at the temperature, pressure, and H/O ratio of the treating atmosphere. If operated in the sooting range, the cracks, which are rich in nucleation sites, gather soot and cannot be cleaned.
While the aforementioned fluorocarbon cleaning process eliminates passivating oxides from a metallic surface, the fluorination potential of the process is lower than is required to significantly remove protective coatings, bonding layers, and the like which are common on gas turbine engine component parts. The current and prevailing acid bath procedure for removing aluminide-type coatings is undesirable because the procedure is expensive, lengthy, somewhat dangerous, and produces large volumes of hazardous waste. Thus, a clear need exists for an efficient, safe, and clean method for removing aluminide-type coatings from gas turbine engine component parts.